CodeGenerator.php

class CodeGenerator {
    public static $baseCharacters = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";

    public static function increment($x, $digits)
    {
        $charList = static::$baseCharacters;

        // completa cadeia de caracteres com o numero de digitos parametrizados
        $x = trim($x);
        if(strlen($x) < $digits) {
            $x = str_pad($x, $digits, substr($charList, 0, 1), STR_PAD_LEFT);
        }

        $result = preg_split("//", $x, -1, PREG_SPLIT_NO_EMPTY);
        // percorre a cadeia de caracteres de tras pra frente e incrementa o primeiro caractere possivel
        for ($i = $digits - 1; $i >= 0; --$i)
        {
            $char = $result[$i];
            $currentChar = strpos($charList, $char);
            $nextCharPosition = $currentChar+1;

            if($nextCharPosition < strlen($charList)) {
                $nextChar = substr($charList, $nextCharPosition, 1);

                $result[$i] = $nextChar;
                break;
            }
        }

        return implode('', $result);
    }

    public static function decrement($x, $digits)
    {
        $charList = static::$baseCharacters;

        // completa cadeia de caracteres com o numero de digitos parametrizados
        if(strlen($x) < $digits) {
            $x = str_pad($x, $digits, substr($charList, 0, 1), STR_PAD_LEFT);
        }

        $result = preg_split("//", $x, -1, PREG_SPLIT_NO_EMPTY);
        // percorre a cadeia de caracteres de tras pra frente e decrementa o primeiro caractere possivel
        for ($i = $digits - 1; $i >= 0; --$i)
        {
            $char = $result[$i];
            $currentChar = strpos($charList, $char);
            $previousCharPosition = $currentChar-1;

            if($previousCharPosition > -1) {
                $previousChar = substr($charList, $previousCharPosition, 1);

                $result[$i] = $previousChar;

                // define os caracteres apos o decrementado para o ultimo caractere. ex: 3[00] > 2[99]
                for ($j = $i + 1; $j < $digits && $i < $digits - 1; ++$j)
                    $result[$j] = substr($charList, strlen($charList)-1, 1);
                break;
            }
        }

        return implode('', $result);
    }
}